DOUGH: Sift flour and sugar into a medium-size bowl. Cut in butter or margarine until mixture resembles coarse crumbs. Add egg, egg yolk, lemon rind, and salt; mix with a fork to form dough. Cover dough and let rest in refrigerator 20 minutes. Roll out dough into a circle; place in an ungreased springform pan. Form a 3 cm high rim. Bake in preheated 180ºC . oven for 10 minutes. FILLING: Meanwhile, clean and halve grapes, and remove seeds if necessary. Beat egg whites until stiff; blend in sugar, lemon juice, and ground almonds. Carefully fold in the grapes. Remove cake from the oven. Fill baked cake shell with grape mixture, return to the oven, and bake for another 30 minutes at 180ºC . Remove cake from pan and cool on wire rack.